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Design >> RF Design >> Input matching of common-source LNA (with small Cgs)
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Message started by Berti-2 on Jul 13^th, 2017, 10:57pm

Title: Input matching of common-source LNA (with small Cgs)
Post by Berti-2 on Jul 13^th, 2017, 10:57pm

Hi all,

I am working on a common-source LNA with inductive degeneration.
For the input matching, I followed the standard approach:
Set the real part of the input impedance (Ldeg*gm/Cgs) to 50Ohm and then try to cancel the imaginary part (Cgs*(Lg+Ldeg)*w²-1)/Cgs*w with a series inductor Lg .

In my design, Cgs is very small (~15fF) which results in a large Lg (>100nH).
Can I do something to obtain more reasonable Lg?

Thanks!

Title: Re: Input matching of common-source LNA (with small Cgs)
Post by Jacki on Jul 18^th, 2017, 6:23am

you can put a Cp (parallel capacitor with your common source transistor) to increase the Cgs artificially.

Title: Re: Input matching of common-source LNA (with small Cgs)
Post by Berti-2 on Jul 18^th, 2017, 11:02pm

Thanks. I found the same statement in Razavi's RF microelectronics book :)